The invention relates to the manufacture of integrated circuits, and is directed to a method of improving photoresist ash selectivity. It has particular applicability where a dielectric layer is prone to attack by the ashing process.
In the manufacture of integrated circuits, the technique of photolithography may be required to form circuit patterns. In the practice of this technique, a semiconductor wafer structure is coated with a photoresist. The photoresist is then exposed to ultraviolet radiation which is passed through a patterned template or a mask so that a desired pattern is imaged on the photoresist. This causes changes in the solubility of the exposed areas of the photoresist such that after development in a suitable developer a desired pattern is fixed on the wafer structure. Etching of the pattern in the wafer structure may then occur, and deposits may be made in the etched away areas. After etching or subsequent steps, it is necessary to strip the photoresist from the wafer as it has already served its useful purpose and remove the etch deposits.
In a process as described above, a photoresist layer is coated onto a first layer. The first layer is disposed on a second layer. After UV exposure and development of the photoresist, the photoresist and first layer are etched to produce openings in the first layer which expose the second layer.
An example of such a process to which the present invention is particularly applicable is where the second layer is a dielectric which is prone to attack by the ashing process. In such a situation, it may be necessary to use a protective layer (the first layer) between the dielectric layer and the photoresist. However, even with such a protective layer, after etching of the photoresist and the protective layer is effected to produce openings, the dielectric layer will be exposed through such openings. During ashing, the exposed areas of the dielectric layer may be attacked to create a region of undercut in the dielectric layer. This is undesirable, since small deviations in the etched profiles can adversely affect the performance, yield and reliability of the final integrated circuit.
A process to which the invention may be advantageously applied is where the dielectric layer is made of a low dielectric constant material (low-k). New low-k materials are presently being investigated for their potential use as insulators in semiconductor chip fabrication in conjunction with copper to form high speed/low capacitance interconnections within the semiconductor chip. The two materials are integrated in a Damascene (or dual-Damascene) structure where copper is deposited in etched patterns within the low-k material to form an embedded-dielectric architecture.
In one embodiment of this process, a hard mask is present between the low-k dielectric layer and the photoresist. After the photoresist is exposed and developed, it and the hard mask are etched, after which the photoresist layer may be stripped. Since most of the low-k materials presently being investigated are organic polymers or inorganic materials with weak bonds, they are prone to unwanted attack by conventional oxygen based dry clean processes. Typically the organic low-k materials are removed at roughly the same rate as photoresist, making the ash selectivity of photoresist to low-k materials close to unity (selectivity is defined as the rate of removal ratio between the two materials). As discussed above, this can cause a problem in that undercutting of the dielectric material may occur. The inorganic materials while not substantially etched are typically damaged by the plasma to the point that their dielectric constant increases to above 3.0.
In accordance with the present invention, an improved method of stripping a photoresist layer is provided wherein the photoresist is coated on a first layer except where there are openings in the first layer which expose a second layer, which comprises the steps of, applying a re-coating material on the photoresist layer having ash characteristics similar to the material of the photoresist layer, which re-coating material extends through and fills the openings in the first layer, ashing the stack comprised of the photoresist layer and re-coating material, and removing such re-coating material as remains in the openings in the first layer after the ashing.